The Critical Period for Control of Sex in Moina

Abstract

Banta and Brown 1 have shown that with crowding under favorable food conditions the critical period for sex control with Moina macrocopa is approximately 4 hours before the eggs are laid into the brood chamber. Stuart, Cooper and Miller, 2 while unable to affect control of the sex of the same Cladoceran species later than the fourth hour before egg laying, concluded that sex could be influenced by conditions prevailing previous to the fourth hour before egg laying. The following experiment was designed to show that the critical period for sex control is dependent to some extent upon the environmental conditions under which the animals are reared, i. e., upon environmental conditions present earlier than, and interrupted before, the time of the critical period as described by Banta and Brown for crowding under good food conditions.

In 4 series of bottles, A, B, C and D, each series containing from 15 to 20 bottles, were placed 75 ml. of varying dilutions of manure infusion and 10 young Moina macrocopa females. The infusion in series A and B was diluted 12.5 times, in series C, 25 times, and in D, 100 times. (Dilutions were made with pond water.) All bottles were then incubated at approximately 25°C. Series A served as a control and was not manipulated in any way. Approximately 15 hours before the egg-laying period of the mothers in series B, C, and D the crowded mothers from one bottle in each series were isolated, each mother being transferred to a separate bottle containing 25 ml. of normal medium (concentrated manure infusion diluted 12.5 times). One hour later the same procedure was repeated with another bottle from each series and again another 1 hour later, and so on every hour until the mothers in the different series laid eggs.